1. Field of the Invention
This invention relates to a system and method of interference cancellation for use with respect to a mobile station having a smart antenna and a Rake demodulator.
2. Description of Related Art
The capacity of wireless Code Division Multiple Access (CDMA) systems in the forward link direction (i.e., from a base station to a mobile receiver) is limited by both intra-cell and inter-cell cochannel interferences. In particular, when the mobile unit is close to a cell boundary, the desired signal is disturbed by relatively strong interference from neighboring base stations. Antenna arrays have been previously suggested for base stations of CDMA systems to improve the capacity in the reverse link through space diversity and interference cancellation. Less attention is given to the forward link due to the reliance on orthogonal spreading codes to handle the cochannel interference. The performance in the forward link is limited, however, by multipath fading and inter-cell interference.
In IS-95 CDMA, signals from the same base station and same path are separated by a set of orthogonal codes (Walsh codes), which eliminate the interference of other users' signals in the same signal path from the home cell (see, for example, J. D. Gibson, The Mobile Communications Handbook, Boca Raton, Fla., CRC Press, Inc., 1996 and T. S. Rappaport, Wireless Communications: Principles and Practice, Upper Saddle River, N.J., Prentice Hall PTR, 1996). The other signal paths from a home base station, however, create self-interference. In addition, signals from different base stations are identified by a special short pseudo random code. Such base stations share the same short code, but with different shifts, and hence due to nonzero autocorrelation there exists inter-cell interference. The worst case occurs at the cell boundary point, where the desired signal is the weakest and the inter-cell interference is the strongest.
Similar to its use in the reverse link, an antenna array at the mobile station can be used as a diversity combiner to maximize the signal-to-interference plus noise ratio (SINR). Due to packaging and cost considerations, however, such an array needs to be small. A dual antenna mobile station for wireless communications has been suggested and its implementation was studied in a paper by M. Lefevre, M. A. Jensen, and M. D. Rice, ("Indoor measurements of handset dual-antenna diversity performance," in IEEE 47.sup.th Vehicular Technology Conference Proceedings, (Phoenix, Ariz.), pp. 1763-1767, May 1997).
The preferred embodiment of the present invention relates to a receiver with a two-element array, referred to as a smart antenna receiver. Adaptive arrays are employed to utilize the known direction of arrival and signal waveform structure of desired signal for interference cancellation in point-to-point communication (see, for example, S. P. Applebaum and D. J. Chapman, "Adaptive Arrays With Main Beam Constrains," IEEE Trans. Antennas Propagat., vol. 24, pp. 650-662, September 1976 and J. R. T. Compton, "An Adaptive Array in Speed-Spectrum Communications," Proc. IEEE, vol. 66, pp. 289-298, March 1978), wherein by using the pointing vector, the desired signal is co-phased and removed, prior to the application of weighting for interference cancellation. To reduce sensitivity to pointing vector error in a point-to-point communication application, a self-correcting loop was suggested to minimize the error by Y. Bar-Ness and F. Haber ("Self-Correcting Interference Cancelling Processor for Point-to-Point Communications," in Proceedings of the 24.sup.th Midwest Symposium on Circuit and Systems, (Albuquerque, N.Mex.), pp. 663-665, June 1981).
In multi-user wireless and other similar communication applications, such a pointing vector is not well defined, and hence cannot be used easily or accurately used by a mobile receiver for interference cancellation.